The present invention relates to extending the tuning range of a voltage controlled oscillator (VCO).
In developing products which are compatible with industry standards, such as GSM/DCS (Global System for Mobile Communications/Digital Communication Systems), it is necessary to tailor the design within the constraints imposed by those standards. For example, an oscillator may be required to have a wide tuning range, while limiting phase noise and tuning sensitivity variation. At times, it can be challenging to achieve all of the requirements given commercially available components and their inherent tolerances. It is particularly difficult to do so with a conventional tank circuit--an inductor and a capacitor in parallel.
Phase noise of the VCO is determined largely by the Q (quality factor) of the tank circuit. A VCO with less phase noise can be achieved if there is a larger Q. This will result in lower total RMS phase error in the system and an output signal with a cleaner spectrum. RMS phase error and spectrum purity are two of the most important specifications for any communications system.
A varactor may be used in designing a tank circuit. The voltage dependent capacitance of the varactor can be varied to tune the VCO frequency. The tank circuit Q is dependent on the Q factors of its components, and the Q of a varactor is typically much lower than that of other capacitances or inductances. When a varactor is present in a tank circuit to tune the VCO frequency, the Q of the varactor is usually the limiting factor of a tank circuit Q.
The degradation of the tank circuit Q caused by a varactor can be reduced by using a coupling capacitance to reduce the impact of the varactor capacitance on performance of the tank circuit. However, this also reduces the tuning range of the VCO. Around a given operating frequency, the tuning range can depend on the capacitance ratio of the tank circuit, which results from the impact on the tank circuit of the variable capacitance of the varactor. Therefore, modifying the impact of the varactor capacitance on the tank circuit by adjusting the coupling capacitance is a trade-off. Extending tuning range degrades Q and phase noise, and vice versa.
In addition, capacitance of a varactor generally varies nonlinearly with respect to the control voltage. Consequently, increasing the impact of the varactor capacitance on the tank circuit also degrades the tuning sensitivity variation of the VCO.
Besides adjusting a coupling capacitance, the tuning range also can be extended by using a varactor with a greater range of capacitance. However, any design is limited by what is commercially available to implement the design. Varactors with a greater range of capacitance generally have a lower Q and more non-linearity. The tuning range can be extended using a varactor with a greater range of capacitance, but the tuning sensitivity and phase noise of the VCO will suffer.
An improved and novel way to extend the tuning range of a VCO while also satisfying the other constraints is to connect a shunt inductance in parallel with the varactor. Prior art did not recognize this way to extend tuning range of a VCO tank circuit without affecting tuning sensitivity variation and phase noise.
The features of the present invention which are believed to be novel are set forth below with particularity in the appended claims. The invention, together with further advantages thereof, may be understood by reference to the following description in conjunction with the accompanying figures. FIG. 3 illustrates an embodiment of the invention.